Identification of O-methyl-(−)-epicatechin-O-sulphate metabolites by mass-spectrometry after O-methylation with trimethylsilyldiazomethane

(−)-Epicatechin, an abundant dietary polyphenol found mainly in cocoa and tea, is known to extensively undergo metabolism after ingestion giving rise to a complex series of conjugated metabolites including numerous isomers. In the present study, the combination of fractionation, chemical derivatization and various mass spectrometric approaches is described to determine the exact position of sulphate group in methylated epicatechin metabolites. Four O-methyl-(−)-epicatechin-O-sulphate metabolites isolated from human urine samples were derivatized under mild condition using trimethylsilyldiazomethane (TMSD) in the presence of methanol. The resulting methylated reaction products were then analyzed by high resolution and multistage mass spectrometry for the subsequent identification of the sulphate positional isomers. Results show that O-methylation affects the charge delocalization in negatively charged ions and hereby the fragmentation pattern of the sulphate isomers allowing the identification of diagnostic ions. In addition, this study demonstrates that methoxy derivatives of polyphenol metabolites can be prepared using TMSD. Subsequently, the localization of the sulphate group in the polyphenol metabolites can be achieved by analyzing the methoxy derivatives by multistage mass spectrometry. Using an enzymatic reaction for identification of the O-methyl position, and a chemical O-methylation with TMSD follow by high resolution and multistage tandem MS for the identification of the sulphate group position, we were able to identify the previously unknown O-methyl-(−)-epicatechin-O-sulphate. Accordingly, we identified 3′-O-methyl-(−)-epicatechin-5-O-sulphate and 3′-O-methyl-(−)-epicatechin-7-O-sulphate as the main O-methyl-(−)-epicatechin-sulfates(−)-epicatechin metabolites in humans.

► (−)-Epicatechin is known to extensively undergo metabolism after ingestion.
► Localization of sulphate group at carbon 5 or 7 by mass spectrometry (MS) is a challenge.
► This analysis can be achieved by analyzing methyl derivatives by multistage MS.